1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, such an apparatus provided with a fixing device for fixing a toner image formed on a recording material in the form of a paper for image formation processing thereon, and a feeding device for feeding or conveying the paper to the fixing device and ejecting it.
2. Description of the Related Art
In a known image forming apparatus, a first and a second thermistor are arranged for detecting the temperature of a central portion and one end portion, respectively, of a heating roller of the fixing device so that the temperatures of the entire heating roller can be controlled to be kept within an allowable range. Such an image forming apparatus, however, when printing is continuously effected on sheets of paper of a narrow width such as A4R, B5 and A5, a central portion of the heating roller is deprived of heat by the paper in contact therewith, so that the temperature thereof lowers. Then, if the heating roller is heated to recover the temperature lowered, the temperature at the end portions of the heating roller from which heat is not deprived might go up too much. The technology disclosed in Japanese Patent Application Laid-Open No. 7-199694 for instance is known as a proposal for solving this problem. This image forming apparatus feeds or conveys sheets of paper in such a manner that the center line of each sheet passes the center of heating roller, with the paper feed timing being changed according to the width of the paper. For instance, when printing on a paper of a narrow width such as an envelope, the temperature at the end portions of the heating roller is prevented from rising to a high temperature by decreasing the paper feed timing or rate from a conventional rate of 34 pieces per minute to 2.5 pieces per minute.
Since the above-mentioned known image forming apparatus changes the paper feed timing in accordance with the width of the paper to be printed, the printing speed for a narrow paper is decreased uniformly. Therefore, though it is not desirous that the temperature at the end portions of the heating roller exceeds the allowable range too much frequently, there is a demand that the paper frequently used (i.e., the amount of use is large) among narrow papers is to be processed or printed at high speed.
The present invention is intended to solve the above-mentioned problems and has for its object to provide an image forming apparatus in which the printing speed is able to be changed according to the demand of the user. In addition, another object of the present invention is to provide an image forming apparatus which is capable of processing continuous printing as efficiently as possible in a range where the temperature of a fixing device does not exceed an allowable temperature range.
In order to solve the problems as described above, the present invention resides in an image forming apparatus for forming an image on a paper, which comprises: a fixing device for heating an image transferred to a paper to fix it thereon; a feeding device for feeding the paper to the fixing device; a setting device for setting a mode of paper feed intervals of the feeding device; and a controller for changing the paper feed intervals of the feeding device based on the mode set by the setting device.
According to such a construction, the paper feed intervals can be changed based on a mode which is set by the user, etc., thus making it possible to fulfill the user request for processing speed during continuous printing. In a preferred form of the present invention, the fixing device comprises a pressure roller 6 and a heating roller 7, and the feeding device comprises a paper feed path 3, and rollers such as paper feed rollers 2, a register rollers 4, etc., arranged along the paper feed path. In addition, the setting device for setting a mode comprises a console panel 14, and the controller comprises a CPU 20.
Further, in the image forming apparatus of the present invention, the modes to be set by the setting device include at least either one of a first mode in which the paper feed intervals are changed based on the size of a paper and a second mode in which the paper feed intervals are changed based on the using frequency of a paper size.
In case of a small paper size, it is difficult to absorb heat at the end portions of the fixing device as compared with the central portion thereof, and hence when the fixing device is heated based on the temperature of the central portion thereof, the temperature at the end portions of the fixing device is easy to rise, thus exceeding an allowable range easily as compared with the temperature at the central portion thereof. In contrast, in case of a large paper size, heat at the end portions of the fixing device can be easily absorbed like at the central portion thereof, and hence the temperature at the end portions of the fixing device does not exceed the allowable range easily. Therefore, with the provision of such a mode (the first mode) in which the paper feed intervals can be changed in consideration of the paper size, there can be achieved such a construction as to prevent the temperature at the end portions of the fixing device from exceeding the allowable range without reducing the efficiency in continuous printing. Moreover, in case of a high using frequency, it is possible to efficiently perform processing as a whole if the paper feed intervals can be reduced, whereas in case of a low using frequency, even if the paper feed intervals are increased, there will be no much influence on reduction in the overall efficiency in continuous printing processing (continuous image formating processing). Accordingly, in case of the low using frequency, increasing the paper feed intervals serves to prevent the temperature at the end portions of the fixing device from rising beyond the allowable range. Thus, according to the present invention, efficiency in continuous printing can be improved.
Moreover, according to the present invention, in the first mode, the paper feed intervals are set to be smaller when the paper size is large than when the paper size is small.
In case of a small paper size, heat is more difficult to be aborbed at the end portions of the fixing device than at the central portion thereof, and hence when the fixing device is heated based on the temperature of the central portion thereof, the temperature of the fixing device rises more easily to exceed an allowable range at the end portions thereof than at the central portion thereof. In contrast, in case of a large paper size, heat at the end portions of the fixing device can be easily absorbed like at the central portion thereof, and hence the temperature at the end portions of the fixing device does not exceed the allowable range easily. Therefore, it is possible to set the paper feed intervals smaller when the paper size is large than when the paper size is small, thereby increasing the speed and efficiency in continuous printing processing.
Furthermore, according to the present invention, in the second mode, the paper feed intervals are set smaller for the paper of a high using frequency than for the paper of a low using frequency.
When the using frequency is high, reducing the paper feed intervals serves to efficiently perform processing as a whole. On the other hand, when the using frequency is low, increasing the paper feed intervals does not influence reduction in the overall efficiency in continuous printing processing so much. Thus, in case of a high using frequency, by reducing the paper feed intervals, continuous print processing is sped up for improved efficiency as a whole.
In addition, the image forming apparatus of the present invention further comprises a temperature detector for detecting the temperature at the end portions of the fixing device. The controller stops the feeding of paper by the feeding device based on the temperature at the end portions of the fixing device detected by the temperature detector.
Further, in the present invention, the mode set by the setting device includes a mode in which the paper feed intervals are set to be smaller when the paper size of the paper is large than when the paper size of the paper is small, and in which when the temperature of the fixing device exceeds a predetermined value, the controller stops, based on the temperature at the end portion of the fixing device detected by the temperature detector, the feeding of the paper by the feeding device until the temperature of the fixing device falls to or below the predetermined value.
Furthermore, in the present invention, the mode set by the setting device includes a mode in which the paper feed intervals are set to be smaller when a paper of a size has a designation of a high using frequency than when a paper of the same size has no designation of a using frequency, and in which when the temperature of the fixing device exceeds a predetermined value, the controller stops, based on the temperature at the end portion of the fixing device detected by the temperature detector, the feeding of the paper by the feeding device until the temperature of the fixing device falls to or below the predetermined value.
According to the constructions as described above, the temperature at the end portion of the fixing device can be prevented from exceeding the allowable range while improving the efficiency in continuous printing processing.
Still further, an image forming apparatus according to the present invention comprises: a heating roller for fixing a toner image formed on a paper to the paper so as to perform processing of image formation on the paper as a recording material and eject the paper; a fixing temperature control device for controlling the temperature of the heating roller so as to be kept within an allowable range; and a controller for controlling paper feed intervals in such a manner that the paper feed intervals are small for a paper having a width greater than a threshold, large for a paper having a width equal to or less than the threshold with no designation of a high using frequency, and small for a paper having a width equal to or less than the threshold with a designation of a high using frequency.
With such a construction, in case of using a paper of a wide width in the fixing process, it is possible to prevent the temperature only at the end portions of the heating roller from rising excessively beyond the allowable range, enabling processing of image formation to be carried out at high speed for improved efficiency. On the other hand, in case of a narrow paper having a low using frequency, as the paper feeding speed increases, the temperature at the end portions alone of the heating roller is liable to rise excessively beyond the allowable range, and hence the narrow paper is fed at low speed even with a decreased efficiency because of the using frequency being low, thus preventing damage to the heating roller, etc. In addition, in case of a narrow paper having a high using frequency, when the paper feeding speed is increased, there occurs a danger that only the end portions of the heating roller become a high temperature exceeding the allowable range, but emphasis is placed on the high using frequency to perform high-speed processing of image formation while permitting the temperature control on the heating roller to be carried out by the fixing temperature control device. Therefore, the efficiency in the image formation processing can be maintained high while reducing damage to the heating roller, etc.
Moreover, in the present invention, the heating roller includes a built-in heater, and the fixing temperature control device comprises: a first temperature detector disposed at a central portion of the heating roller; a second temperature detector disposed at an end portion of the heating roller; and a fixing control circuit for controlling the heater of the heating roller based on the temperatures detected by the first and second temperature detectors in such a manner that the temperatures of the heating roller are kept within an allowable range. Thus, the temperature detection of the heating roller can be carried out by a simple construction using the two thermistors. For instance, the heater can be heated by high-frequency current with good response and high efficiency.
Besides, when the temperature detected by the second temperature detector of the fixing temperature control device exceeds the allowable range during high-speed feeding of a narrow paper having a high using frequency, the controller temporarily stops image forming processing until the detected temperature returns to fall within the allowable range under the control of the fixing temperature control device. Thus, it is possible to cope, in a simple manner, with such a situation in which the temperature at the end portion of the heating roller exceeds the allowable range.